Aqueous coating compositions are known and described for example in U.S. Pat. Nos. 4,794,147; 4,791,168; 4,518,724; and 4,403,055.
These aqueous coating compositions comprise a principal resin, optionally a crosslinker, pigments dispersed in a grind resin and other additives such as solvents, control agents fillers and the like.
Many pigments, especially organics, used in coating compositions for the automotive industry are hydrophobic. These pigments, when used in aqueous coating systems require the use of specialized grind resins with the additions of dispersing agents and solvents to overcome their incompatibility with water and poor ability to grind.
Pigment pastes are usually prepared by dispersing a pigment in a grinding resin in the presence of plasticizers, wetting agents, surfactants or other ingredients in a ball mill, sand mill, cowles mill or continuous mill until the pigment has been reduced to the desired particle size and is wetted by the resin or dispersed in it.
One disadvantage of pigment pastes is that they contain volatile organic compounds (VOC). Also, they often require a large amount of resin relative to pigment, and the resulting pigment pastes are not very concentrated. In addition, pastes made with conventional resins often do not have optimum color development.
It is therefore an object of the present invention to provide a process for the preparation of a pigment dispersant for an aqueous coating composition which does not need the use of volatile organic compounds and which provides stable aqueous pigment pastes or pigment dispersions. Another object is to increase the amount of pigments in the dispersion and simultaneously reduce the amount of resin. Yet another object of the invention is an aqueous coating composition comprising such pigment pastes or pigment dispersions.